The electrosynthesis of sodium chlorate (“NaClO3”) by the electrolysis of sodium chloride solutions in an electrolysis cell has been known in the art for a long time and is generally described by the following reaction:NaCl+3H2O→NaClO3+3H2The reaction proceeds only partially and the resulting aqueous solution of sodium chlorate and sodium chloride is typically referred to as a cell liquor. Sodium chlorate is obtained by selective crystallization from this cell liquor. The remaining liquor after selective crystallization of sodium chlorate is typically referred to as the mother liquor. The mother liquor is recycled to the electrolysis reaction, generally after addition of make-up sodium chloride.
There are several processes occurring in the electrolysis cell that can increase the energy consumption per ton of chlorate produced by lowering cell efficiency and/or increasing cell voltage. For example, the formation of sodium perchlorate (“NaClO4”) in the cell (by direct electrochemical oxidation of sodium chlorate) and its subsequent accumulation affects the function of the anode and reduces the solubility of sodium chloride in the electrolyte.
The formation of sodium perchlorate especially becomes an issue in view of the recent trend in the manufacture of sodium chlorate toward producing the sodium chlorate in closed loop processes so that there is reduced effluent discharge to the plant. In such a closed loop process or closed loop system, the mother liquor produced from the cell liquor by the removal of crystalline sodium chlorate is recycled to an earlier point in the process or system. Significant amounts of sodium perchlorate can accumulate in closed loop processes for the production of sodium chlorate.
Methods for the removal of sodium perchlorate are known in the art. For example, the addition of potassium chloride results in removal of the perchlorate in the form of potassium perchlorate. However, the addition of potassium chloride represents a significant operating expense for the sodium chlorate production process. Accordingly, there is still a need in the industry for improved methods for removal of perchlorate from an electrolytic process for producing sodium chlorate.